Distorted growth on citrus trees is often caused by Citrus leafminer (Phyllocnistis citrella). The adults are tiny, night-flying moths with wingspans of only about 5mm. The females lay their eggs on new young growth in the warmer months of the year. When the larvae hatch, they tunnel into the leaves, causing silvery ‘squiggles’ on the foliage. The damaged leaves are often twisted and curled. The flat, yellow larvae are around 3mm long. They pupate inside a rolled up leaf, and under optimum conditions their whole life cycle takes from two to three weeks. This pest affects both native and cultivated citrus, and is found in most of the citrus growing areas of Australia.
The registered control for Citrus leafminer is PestOil. This is a highly refined petroleum oil which should be sprayed as per directions during flush growth periods (usually summer and autumn). PestOil does not kill the tiny moths, but leafminer numbers are reduced because the moths don’t like laying their eggs on oily leaves. Petroleum oil sprays are relatively safe to use. They are not as harmful to beneficial insects as other chemicals, and they are low in toxicity to humans. However, another option is just to prune off the unsightly foliage, put it in a plastic bag and dispose of it in the garbage. If you would rather not use any sort of control, you don’t need to – your trees will still grow and produce fruit.
Hortex PestOil is available at nurseries, hardware stores and supermarkets nationwide. Expect to pay about $13.95 for a 750ml bottle.